So many things in life are transitory and short-lived. While we know God is permanent, stable, and dependable, yet we often still feel unsettled. So this verse serves as an anchor to our storm-tossed souls, reminding us of the faithfulness of God and His Word, as Spurgeon and others have noted:

After tossing about on a sea of trouble the Psalmist here leaps to shore and stands upon a rock. Jehovah’s word is not fickle nor uncertain; it is settled, determined, fixed, sure, immovable. Man’s teachings change so often that there is never time for them to be settled; but the Lord’s word is from of old the same, and will remain unchanged eternally. Some men are never happier than when they are unsettling everything and everybody; but God’s mind is not with them. The power and glory of heaven have confirmed each sentence which the mouth of the Lord has spoken, and so confirmed it that to all eternity it must stand the same, — settled in heaven, where nothing can reach it. In the former section David’s soul fainted, but here the good man looks out of self and perceives that the Lord fainteth not, neither is weary, neither is there any failure in his word.…

So Spurgeon concisely helps us to think about the key elements of this verse:

Consider,
1. The term, “thy word.”
(a) A word is a revealed thought. The Scriptures are just this: the thoughts and purposes of God made intelligible to man.
(b) But a “word” also marks specially unity (it is one word) and wholeness or completeness, a word, not a syllable. The Scriptures are one and complete.

2. The statement, “for ever settled in heaven.”
(a) “Settled in heaven” before it came to earth; therefore it could come as a continuous unfolding, through various dispensations, without the shadow of hesitation or contradiction manifest in it.
(b) Abides “settled in heaven, “for its central revelation; the atonement is a completed fact, and Christ is now in heaven a perfected Saviour; thus the word is unalterable.
(c) “For ever settled in heaven.” Not only because God in heaven is of one mind and cannot be turned; but because righteousness itself, the righteousness of heaven, demands that an atonement by suffering shall be fully and everlastingly answered by its due reward.

3. The lessons.
(a) If settled in heaven, men on earth can never unsettle it.
(b) The wicked may not indulge a future hope arising from any new dispensation beyond the grave; God’s present word to us cannot then be unsettled.
(c) The godly may rely on a settled word amidst the unsettled experiences and feelings incident to earth. — J.F.

He also offers the thoughts of others on this verse:

If we look at God’s word of promise, as it is in our unsettled hearts, we dream that it’s as ready to waver as our hearts are; as the shadow of the sun and moon in the water seems to shake as much as the water doth which it shines upon. Yet for all this seeming shaking here below, the sun and moon go on a steadfast course in heaven. So the Psalmist tells us that however our hearts stagger at a promise through unbelief, nay, and our unbelief makes us believe that the promise often is shaken; yet God’s word is settled, though not in our hearts, yet “in heaven”; yea, and there “for ever, “as settled as heaven itself is; yea, more than so; for “heaven and earth may pass, “but “not one jot or tittle of the law (and therefore of the gospel) shall fail”: Luke 16:17. — Anthony Tuckney, 1599-1670.

Whenever you look to heaven, remember that within you have a God, who hath fixed his residence and shown his glory there, and made it the seat both of his mercy and justice. You have also there a Saviour who, after he had died for our sins, sat down at the right hand of Majesty, to see his promises accomplished, and by his word to subdue the whole world. There are angels that “do his commandment, hearkening to the voice of his word”: Psalm 103:20. There are glorified saints, who see God face to face, and dwell with him for evermore, and came thither by the same covenant which is propounded to us, as the charter of our peace and hope. In the outer region of heaven we see the sun and moon, and all the heavenly bodies, move in that fixed course and order wherein God hath set them; and will God show his constancy in the course of nature, and be fickle and changeable in the covenant of grace, wherein he hath disposed the order and method of his mercies? — Thomas Manton.